The production of paper begins with the processing of wood. Wood is chiefly composed of two major substances; both are organic, that is, their molecules are built around chains and rings of carbon atoms. Cellulose, which occurs in the walls of the plant cells, is the fibrous material that is used to make paper. Lignin is a large, complex molecule; it acts as a kind of glue that holds the cellulose fibers together and stiffens the cell walls, giving wood its mechanical strength. In order to convert wood into pulp suitable for making paper, the cellulose fibers must be freed from the lignin. In mechanical pulping this is done by tearing the wood fibers apart physically to create groundwood pulp, leaving most of the lignin intact in the pulp. The high lignin content of groundwood pulp leaves the paper products weak and prone to degradation (e.g. yellowing) over time. Mechanical pulp is used principally to manufacture newsprint and some magazines.
In most pulp production lignin is separated from the fibers chemically. For example, in the kraft process, wood chips are heated (“cooked”) in a solution of sodium hydroxide and sodium sulfide. The lignin is broken down into smaller segments and dissolves into the solution. In the next step, “brownstock washing,” the breakdown products and chemicals are washed out of the pulp and sent to the recovery boiler. Kraft unbleached pulp has a distinctive dark brown color, due to darkened residual lignin, but is nevertheless exceptionally strong and suitable for packaging, tissue and toweling.
For brighter and more durable products the pulp must be bleached. In the bleaching process, the color in the residual lignin is either neutralized (by destroying the chromophoric groups) or removed with the lignin. This process traditionally has been accomplished for kraft pulp by chlorine bleaching, usually followed by washing and extraction of the chemicals and breakdown products. This process is not much different than washing clothes, the stains imbedded in cloth fibers are either neutralized by bleach, or broken down and washed out.
In current pulp production processes, the lignin solution typically undergoes two or more separate washing operations. For example, the groundwood or wood chips are first processed with chemicals under pressure and temperature, usually by either the kraft process or by the sulfite acid process. In either process, digestion dissolves the lignins thereby freeing the fibers and placing the lignin components into solution. In both processes the resulting liquid is dark in color, and the residual liquid which does not drain from the pulp and the remaining contaminants must be washed from the pulp. Further, it is desirable to recover spent liquid at as high a concentration as practical to minimize the cost of the subsequent recovery of chemicals.
Brown pulp which has been so washed retains a definite brown color and the pulp which remains is usually too highly colored for making white paper. Also, if any lignin is present, paper made from such pulp may not have a high degree of permanence and will yellow in time. Therefore, it is common and conventional to apply a bleaching process to the pulp, not only to improve whiteness, but to improve permanence of the whiteness.
Bleaching may not be accomplished in a single stage and may be performed in two or more stages, each followed by washing. After bleach treatments, the pulp is subjected to a washing action to remove the water which contains the spent bleaching agents and dissolved lignin.
One particular type of industrial fabric, which is used in such application, is the pulp washing fabric, which is used, for example, in the Black Clawson Chemi Washer.
U.S. Pat. No. 5,275,024 shows an example of a current belt-type pulp washing machine that includes a dewatering stage (or “formation zone”) and multiple counter-current washing stages (or collectively “displacement zone”). The machine employs an endless moving foraminous belt which extends about a breast roll defining an on-running end and a couch roll defining an off-running end, with a generally horizontal upper run of the belt extending between the rolls. A series of suction boxes located underneath the belt provides for initial dewatering of the pulp in the formation zone, and is combined with a series of showers to provide washing and dewatering of the pulp in the displacement zone.
The machine downstream from the headbox and the forming zone is divided into a series of washing zones or stages to which a washing liquid is applied from above for drainage through the pulp mat. The freshest or cleanest washing liquid is applied to the zone nearest the off-running end of the wire and the liquid drained through the mat at that zone is collected from the suction boxes and delivered to the immediately preceding washing zone. This is repeated from zone to zone, so that the cleanest pulp is treated with the cleanest water, and the dirtiest pulp is treated with the dirtiest water.
In most pulp washing applications, it is desirable to use tensioned fabrics, which are supplied with pin seams for ease of installation. This use of pin seams in these types of products also allows machine manufacturers to produce less expensive non-cantilevered washing systems. The problems with pin-seamed products primarily revolve around issues of strength relative to endless woven or endless seamed alternatives. Specifically, the seam area in a fabric has lower strength than the main fabric body. Depending upon the design of the fabric, the seam strength can be as low as 50% of the fabric body tensile strength. Thus a seam, which is a desirable feature, is the weakest portion of the fabric. As most pulp washing systems (vacuum slotted decks) offer the potential for high fabric wear side abrasion, seams or seam components, which are typically thicker in caliper that the body of the fabric, can experience preferentially higher wear rates resulting in seam strength reduction and premature failure (seam breaks).
To mitigate this wear-based failure, it has become a standard practice to provide some sort of sacrificial wear surface as a protective barrier to extend seam life. U.S. Pat. No. 5,791,383 describes a practice in which terminal ends from the seaming process are purposely left uncut to cover the seam area. While somewhat effective, this practice can make field installation a difficult endeavor.
An alternative practice, which does not adversely affect field installation, is the use of a CD wear bead or strip of polymeric material on either side of the seam. FIG. 1 shows a fabric 10 including a seam 16 formed of loops 12 and at least one pintle 14. The fabric 10 also includes a wear beads/strip 18. The wear beads 18 are typically placed within 10 cm, on either side of the seam 16, and are thicker in caliper than the seam 16.
The use of the wear strips 18 theoretically allows the seam 16 to essentially be free of wear until such time as the bead/strip is abraded to the caliper of the seam and seam abrasion begins. However, because of the continuous nature of these CD wear beads/strips 18, there is a high potential for catastrophic failure of the bead/strip as a result of either concentrated force along a common plane or peeling. The shear force to remove a bead/strip 18 is typically on the order of 20 times the peel strength in the cross direction. Thus, any imperfection in the wear bead/strip deposition, or any sections of the wear bead/strip that become locally damaged during pulp processing results in the wear bead/strip strength being reduced to the peel strength. Such imperfections can be caused during the manufacturing process or caused by delamination damage anywhere along the length of the bead material deposited across the width of the fabric. These imperfections ultimately result in ineffective wear protection that fails early in the fabric run.
Accordingly, the present invention is directed to overcoming these shortcomings of the prior art fabrics.